The primary anaerobic digester processes on the market, that are available for industry/agriculture/municipal waste treatment and environmental remediation, include digesters which are either in ground or above ground. In addition, they operate generally at the psychrophilic or mesophilic temperature ranges; however, there are some thermophilic units in operation. The psychrophilic units operate normally at a temperature of 18° C. (65° F.) and the mesophilic units at a temperature range of 35° C. to 40.5° C. (95° F. to 105° F.). There are many variations with these designed anaerobic systems, and each has certain advantages. Detailed below are general descriptions of the three major classes of digester with a further explanation as to some of their critical limitations.
The least expensive digester is an in ground system, such as a lagoon, which is generally an excavated shallow basin with a very large surface area. The sidewalls can be earthen and may include flexible membrane covers. There is generally no agitation in these units and most operate at ambient temperature, i.e.—psychrophilic. In-ground systems (i.e. lagoons) have certain intrinsic problems such as dilution of the process from either groundwater beneath the lagoon or rainwater from the sides or top. Rainwater and snow/ice that accumulates on the flexible membrane cover will depress the cover in certain areas and cause the biogas to collect in pockets. Unintended water within the process dilutes the organic matter, and disrupts the digester temperatures within the unit, affecting the performance and efficiency. High winds and UV radiation also cause problems, damaging the flexible covering. Given the geometry of these lagoons, which are relatively shallow with a large surface area, evaporation combined with the lack of uniform agitation, causes a significant and inevitable accumulation of inorganic and heavy organic matter. Generally, these systems are shut down annually for over a month to allow for solids removal and subsequent restart of the process. Maintaining proper operating temperature is recognized as a technical problem endemic to these units. This lack of temperature and exposure time results in a marginal and unpredictable pathogen kill. Most if not all in ground lagoons require a secondary lagoon in which the processed feedstock needs up to an additional 180 days to complete the process to meet nutrient management requirements. Additional aerobic composting in windrows may also be required.
Some in-ground units may incorporate concrete channels, which are laid out in either a long linear fashion or are in a U-shaped configuration. These units typically have concrete walls with concrete lids (or flexible membrane covers) and are built into the ground to retain process heat. These units are generally mesophilic. Heating is typically provided by heating coils or pipes installed either under the digester concrete channel or in the central concrete wall, which separates the two adjacent channels of the U-shaped configuration. In all cases, the heat is transferred by a combination of conduction and convection through the wall and then across the full width of the plug flow within the channel. The hydraulic residence time (HRT), which is the duration for which material to be digested will remain in the digester, ranges from 18 to 28 days. This long duration time necessitates a long digester chamber length and/or a slow throughput which in turn introduces mechanical difficulty providing proper and uniform agitation along the full length of the digester. As a result, without consistent agitation, heating is not uniform and hot and cold areas develop along the length of the digester. Negatively impacting digester performance as measured by throughput, volatile solids destruction, methane gas production and pathogen kill rates. In addition, the lack of uniform agitation along the length of the digester results in the accumulation of inorganic and heavy organic materials that have been introduced into the digester. It should be noted that although the heavy organic matter can be broken down within the digester, any overlay of inorganic matter above the heavy organic matter (such as sand) may isolate the organic matter from the anaerobes. Over time, and generally within one year, the digester needs to be shut down to remove this accumulation of material. This is necessary as the digester's operating volume slowly decreases, due to this buildup, which, if left unattended, will ultimately blind off and restrict the flow through the digester. When this type of digester is shut down, cleaned out and restarted, up to a month of operating time is generally lost.
The above ground, anaerobic digester systems are normally made of poured in place concrete or steel construction materials and insulated as required. These materials are sturdy and water tight thus eliminating many of the intrinsic problems associated with lagoons such as water and wind. However, heat management is very critical to the efficient performance of this type digester. Generally, these vessels are cylindrical in nature and are approximately 12 m (40 ft.) in diameter, and 12 m to 15.25 m (40 to 50 ft.) in height with a vessel volume of 2500 m3 (88,290 ft3) and greater. In the case of the mesophilic units researched, hot water piping is usually located around the interior circumference of the vessel used as an aid in maintaining the optimal operating temperature. An efficient digester should have uniform temperatures throughout the vessel, within a tight tolerance of +/−1.2° C. or 2 ° F. However, convective and conductive heat transfer alone do not provide for homogenous heating throughout a vessel of this size. Therefore, in order to move heat to the center of the vessel mass, agitation is required. This is normally provided by a top or side mounted unit with blades and sufficient energy to occasionally roll over the vessel contents. Top mounted agitators are usually located off centre with horizontal paddles near the top and bottom of the shaft. These agitators attempt to distribute the heat and achieve more uniform temperatures within the digester; however the flow related process requirements for the digester are compromised. The fresh feedstock is mixed in with the older feedstock very quickly, negatively impacting volatile solids destruction, methane gas production, yield, and pathogen kill.
In addition to these above ground anaerobic mesophilic digester systems, there are similar units (far fewer) operating at the thermophilic temperature range of 44° C. to 70° C. (110° F. to 160° F.). Heat management is even more critical to the efficient performance of this type of digester. Generally, as in the case of the mesophilic type of digester described above, these thermophilic units have similar dimensions, capacities and heating/agitation systems. Due to the higher operating temperature of these other thermophilic units, the quantity and resulting surface area of the hot water piping located around the interior circumference of the vessel is increased. To attain the tight temperature tolerances required for digester efficiency, external heat exchangers may be required as convection and conduction alone may not suffice. The temperature of the hot water must also be increased to accelerate the heat transfer rate. This increased ΔT then leads to localized caking and subsequent insulation of the heating pipes. The intensity or level of agitation must also be increased to aid in the heat transfer and the required tight temperature control demanded by the thermophilic process. This increased agitation has the side effect of causing the methane producing bacteria to become dormant and produce less gas. The flows of the contents through the vessel as a result of the increased agitation will also short circuit the passage of the feedstock through the unit, compromising pathogen kill certainty. This short circuit condition does not permit the feedstock to be held at the higher system operating temperatures for the length of time mandated to achieve pathogen kill levels.
Consequently, many of the systems in use as described above require a secondary vessel to finish the digestion process, adding to the Hydraulic Retention Time (HRT). In the event that a secondary anaerobic digester is not installed, the discharge from these digesters can be dewatered and transferred to storage areas for wind rowing. Wind rowing, which is aerobic digestion, is used to complete the overall digestion process to meet nutrient management requirements. Each incremental process step adds significantly to the overall digestion time duration, as well as project cost, operational cost, and overall area requirements. If these additional process steps are not included, the digester performance (as measured by methane gas production, quality, volatile solids destruction, pathogen kill, hydraulic residence time and the final digestate chemical inertness), will be measurably less than the results from the digester technology as covered by this patent description.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings.